1) Field of the Invention
This invention relates to the electromagnetic control of the flow of an electrically conducting liquid.
2) Description of the Prior Art
Molten metal moves often from one vessel to another during industrial processes. Whether it is from a melting or holding furnace to multiple molds in a batch casting process or from a ladle to a tundish to a mold in a continuous caster, in both the ferrous and non-ferrous industries, the control over the flow of the metal is important or key to the process.
The growth of continuous casting in the United States, the emphasis on "clean steel", the rise of ladle and tundish metallurgy, the trend to higher production machines and the need for precise control in innovative casting processes have all increased the importance of flow control in molten steel pouring in particular. According to the magazine, 33 Metal Producing, 80% of the steel melted in American furnaces (87 million tons in 1991) passes through a slide gate or valve. An engineering manager of Sumitomo Metal America, Inc. was quoted in 33 Metal Producing as stating that if a magnetic field controlled the rate of flow, one could eliminate conventional metering or throttling systems and reduce costs. Inland Steel has recently cited the possibility of the use of electromagnetic force to reduce the alumina clogging problem in slide gates. In the steel ingot, casting and non-ferrous metal industry, a similar need is felt. The combined aluminum and copper production in the United States in 1991 was approximately six million tons.
Today, the state-of-the-art of molten metal flow control in industrial processes continues to be by mechanical devices. Three major types of conventional flow control devices are used at the discharge of a furnace, holding vessel or tundish: a metering nozzle, a stopper rod or a slide gate. A metering nozzle is a specially contoured hole through a ceramic block. For a gravity driven flow, the flow rate is simply proportional to the square root of the head of the molten metal above the nozzle and to the square of the nozzle throat diameter. The stopper rod is basically a blunt ended rod suspended above a nozzle and configured with a manual or automatic mechanical means for raising and lowering. The flow rate can be varied from fully open to fully closed using a stopper rod. The slide gate is primarily a hydraulically operated mechanism that basically consists of several stacked ceramic plates, each with a central hole therethrough. The holes may be aligned to allow the flow or misaligned to stop the flow. Both linear and rotary versions are available. Slide gates are predominately used on furnaces and ladles because of their ability to hold high heads for long periods of time.
A particularly critical flow control location of great practical significance is from the tundish to the mold in a continuous casting machine for making steel. As schematically shown in FIG. 1A, a tundish 4 is an intermediate, shallow vessel that provides several functions in a continuous casting machine. Receiving molten metal from a transfer ladle from the furnace, the tundish 4 distributes the molten metal through multiple bottom openings to individual molds. Multiple ladles may be sequenced using the capacity of the tundish as a reservoir. Also, the tundish provides a residence time to allow metal inclusions to float out. According to a 1986 survey, the metering or free flow nozzle is used on approximately half of the total tundishes in the overall United States steel industry; while stopper rods and slide gates are each used on about one-quarter of the steel tundishes, respectively.
One of the prime functions of a tundish is to provide a controlled, uniform flow. A rough stream has a higher surface to volume ratio and, hence, a higher propensity to reoxidize by direct contact with the air. Further, a rough stream will entrain more air and carry it into the mold resulting in disadvantageous turbulence, foaming and sloshing. With a turbulent pool, new steel is continuously brought to the surface for further contact with air. Very little time is left for proper separation of impurities. Also, oxides tend to be thrown to the outside of the mold where they can be trapped in the surface of the strand. Excessive turbulence in the molten crater of the strand can also be a potential cause of a breakout through the shell.
Both stopper rods and slide gates tend to produce rough streams. Also, slide gates and stopper rods are both subject to clogging when casting aluminum killed steels. Toward the end of a sequence cast, the accuracy of flow control gets worse especially with a stopper rod as the flow area between the rod and the nozzle block becomes fouled. Stream flaring occurs in a slide gate 95% of the time during a heat sequence when the slide gate needs to be in its semi-open position. The stream exiting the top portion of the slide gate at an angle translates into a circular motion through and exiting the slide gate.
Metering nozzles also suffer from operational problems. The only way to control flow with a metering nozzle is to control the tundish level height, but this is slow and insensitive being a square root function of the head. Other considerations, such as inclusion float time or vortexing, tend to make changes in tundish level undesirable from a quality standpoint. Generally, for a billet caster, nozzle life limits the sequence length. The nozzle erodes to the point that the flow rate increases over the allowable limit for the machine. Also, clogging tends to limit cast sequencing and, most importantly, the types of steel that may be cast. The only way to stop flow through a metering nozzle is to manually insert a chill plug to freeze the flow. Typically, in the steel industry, this plug must be burned out with an oxygen lance to restart the flow, often damaging the nozzle.
A continuous casting operator is market driven to meet one or more of the following needs: (1) to meet the quality specifications of the grades being cast; (2) to diversify by moving into casting improved grades of steel; (3) to reduce the current cost of casting a given grade of steel; (4) to increase the yield of prime billets, i.e., to reduce waste; and/or (5) to upgrade machinery as it ages to continue to compete in the market.
An electromagnetic flow control device in lieu of the conventional flow control devices has direct bearing on all of these market drivers. As part of an overall caster control system, it does so in a number of important ways to improve process control, improve quality, increase productivity and reduce cost.
When compared to metering nozzles, an electromagnetic flow control device (1) offers the operator of a billet caster the opportunity to now control the flow through the caster rather than react to it; (2) provides independent control over the casting rate to meet tight specifications on the heat removal rates in all commercial grades; (3) provides a greater degree of control over that of changing the tundish level height which is slow and insensitive; (4) offers independent flow control on each nozzle to compensate for uneven nozzle wear or clogging among the multiple strands in a caster fed from the same tundish; and (5) gives the operator the capability to adjust flow independent of strand motion changes to maintain a constant mold level height which is so important to good quality.
Since 95% of the flow control in the mini mill market is by metering nozzle, these advantages are particularly important. The mini mill sector is no longer just the low cost producer of rebar. Higher quality billets for structural shapes and special bar quality (SBQ) are being produced regularly. The ability to counter the traditional nozzle blockage problem of aluminum killed steels via the controlled flow and heat addition capability of an electromagnetic nozzle opens up markets currently not available to the mini mills. In the mini mill industry, a number of billet casters were put into service originally in the 1960s and 1970s. These machines are in need of upgrades to the current state-of-the-art to compete today.
When compared to stopper rods or slide gates, electromagnetic flow control (1) regulates flow without introducing stream roughness and the subsequent mold turbulence, reoxidation and impurity entrapment; (2) eliminates sites where stream velocity changes abruptly which then causes inclusions to accumulate; (3) eliminates the mechanisms needed to move the rod or plates which are subject to wear and failure; and (4) allows a higher number of sequential casts through increased nozzle reliability and performance which results directly in higher productivity and cost savings.
An electromagnetic flow control device can perform a number of beneficial functions that current metering nozzles, stopper rods or slide gates cannot by: (1) electromagnetically improving the steadiness of the pouring stream eliminating turbulence from the ladle stream and the tundish resulting in improved quality; (2) providing additional heat directly at the nozzle to reduce the tendency for inclusion deposition and the possibility of freezing; (3) applying heat at the nozzle electromagnetically to remelt a strand if it has been deliberately frozen off avoiding the damage typically done by an oxygen lance; (4) giving a greater capacity to deal properly with hot or cold heats; and (5) permitting larger tundishes having a greater depth to minimize vortexing and to maximize inclusion float through offsetting the extra head caused by the larger tundishes.
Electromagnetic flow control is also believed to improve performance over the current mechanical flow control in the following ways: (1) it permits a computer to have better and more responsive control over the metal flow rate through the caster to better match the furnace; (2) it achieves more uniformity from cast to cast by reducing the reliance on individual operator's skill and potentially the number of operators needed; and (3) it reduces costly events, such as breakouts, nozzle lancing and caster turnarounds, with the subsequent upturn in strand yield.
An early reference in the metals industry to the use of electromagnetic flow control was in 1960. It involved the use of a 300 kilowatts, 3,000 hertz induction tundish heating system with two coils, one for main body and one for the spouts. While the body coil primarily provided constant temperature control of the metal, the spout coil provided an additional stabilizing effect on the pouring stream preventing splashing and thus, helping to maintain a "quiet" level in the top of the mold. More recently, Garnier at Grenoble, "Liquid Metal Flows and Magnetohydrodynamics", Progress in Astronautics and Aeronautics, Volume 84 (1981), experimented using mercury with high frequency alternating current (ac) electromagnetic devices to achieve convergent or divergent flows in a vertical molten metal column. Takeuchi et. al., at the 1992 TMS Symposium on Magnetohydrodynamics in Process Metallurgy summarized descriptions of linear and rotary ac motors to control the pouring rate from a vessel. Kirillov and Vitkovsky, at the Nagoya International Symposium of Electromagnetic Processing of Materials (1994), discussed Russian electromagnetic brakes for liquid metal flow regulation.
U.S. Pat. No. 2,707,720 discloses a container with an opening in the bottom near the wall and surrounded by an electric coil. An alternating current applied to the coil forces the molten metal to move away from the opening by an induced magnetic pressure. U.S. Pat. Nos. 3,463,365 and 3,701,357 describe devices where an external current is passed through a liquid metal, which then interacts with an externally imposed magnetic field to generate a force component retarding the flow of liquid metal. U.S. Pat. No. 3,695,334 describes the use of a rotating electromagnetic field to generate rotational motion and a radial pressure gradient in a container with a liquid metal inlet at the outer periphery and exit at the central axis. U.S. Pat. Nos. 4,082,207 and 4,324,266 disclose an alternating current winding and an electrically conductive screen to constrict the jet of molten metal at the outlet of a nozzle. U.S. Pat. Nos. 4,805,669 and 4,947,895 disclose electromagnetic valves with specially shaped internal discharge passageways surrounded by induction coils supplied with a high frequency, alternating current. U.S. Pat. No. 4,842,170 teaches a device with an alternating current electromagnetic coil surrounding a nozzle orifice with a central portion designed to allow eddy currents to flow in certain regions and not in others resulting in an axially directed force to impede the flow. Finally, U.S. Pat. No. 5,137,045 discloses an alternating current electric coil surrounding a descending stream to optimize magnetic pressure versus power loss.
All of the devices cited above operate by combining a magnetic field vector B and an electric current density vector J to generate a body force vector F via the vector Lorentz Law, F=J.times.B. The fields and currents may be alternating (ac) in time and/or in space or steady (dc). The current may be internally generated by induction or by an externally applied electric potential. U.S. Pat. Nos. 2,707,720; 4,082,207; 4,324,266; 4,805,669; 4,947,895; 4,842,170; and 5,137,045 utilize single coils supplied with high frequency alternating current to create a time varying, spatially fixed magnetic field. The time variation of the magnetic field results in an electric field according to Maxwell's Equations. In an electrically conducting fluid, the electric field causes eddy currents to flow in the fluid. As stated above, the interactions of the eddy currents and the imposed magnetic field result in electromagnetic body forces exerted on the fluid. The steady component of the body force effectively confines or retards the flow as desired. The time alternating component is not generally useful since it changes too fast for the fluid to follow. The Takeuchi et al. article and U.S. Pat. No. 3,695,334 describe multi-coil, multi-phase ac devices where the magnetic field moves in a rotary or linear fashion. Here, the body forces from the eddy currents try to make the fluid catch up with the field, analogous to the slip of the rotor in a conventional induction motor. U.S. Pat. Nos. 3,463,365 and 3,701,357 use the variation of applying an external electric potential to generate the current.
Most of the systems described in the technical and patent literature have inherent practical limitations. Given the actual metal types, sizes and flow rates of industrial metal pouring situations, devices which use an individual coil carrying a single phase current, such as described by U.S. Pat. Nos. 4,805,669; 4,947,895; 4,082,207; or 4,324,266, need to operate at high frequencies, typically from several thousand to tens of thousands of cycles per second. This is basically to match the skin depth of field penetration where the eddy currents flow relative to the size of the flow stream. Special power supplies are needed to generate the high frequency current from the standard power line supply. Inductive and capacitive matching is needed between the supply, the coil, and the molten metal load. The real and reactive power that needs to be supplied to the system to overcome the losses of the applied and induced eddy currents is in general very high, often hundreds of kilowatts. Buswork connecting the coil to the supply must be low in resistance and inductance, meaning large parallel copper buswork or coaxial copper cables. Generally, because of the high power, the coil and the power cabling must be water cooled. Cooling the coil is already difficult due to the necessity to have it in very close proximity to the elevated temperature of the molten metal. Some devices, such as described by U.S. Pat. No. 4,842,170, also need to insert a specially shaped, non-conducting plug into the flow passage to direct the metal flow or eddy currents in a special way. Such plugs are subject to erosion, clogging or thermal failure, especially when dealing with molten steel. The transfer of electrical current into a device, such as described by U.S. Pat. No. 3,463,365 for a high temperature molten metal, such as steel, is hampered by the lack of suitable electrode materials. The currents must be high to effect the level of retarding pressure required. The point in a metal pouring process where flow control is desired is unlike that of an electric arc furnace where high losses, heating and arcing can be tolerated. The rotating field device described in U.S. Pat. No. 3,695,334 requires high fluid peripheral velocities to offset practical heads and is not suitable where the flow is desired to be as quiescent as possible. The flow passages in metal pouring are, in general, too small for linear ac pumps to be practical.
All of these factors tend to make these kinds of systems complicated and expensive. These factors have severely limited their practical application for flow electromagnetic control.
Baker, "Design of an Eddy-Current Brake for a Sodium-Cooled Nuclear Power Reactor", AIEE Winter Meeting (1960), New York, N.Y., describes the use of a rectangular directing current (dc) flow brake to retard the flow in a liquid metal reactor after a shutdown or scram. Baker's device includes a flattened section of stainless steel pipe suspended between the poles of a conventional C-shaped, iron electromagnet. The working fluid is sodium. Both the rectangular shape of the flow passage and the stainless steel material for the tube are not readily practical for most metallurgical metal pouring situations. Shercliff, The Theory of Electromagnetic Flow Measurement, Cambridge University Press (1962), describes a variation of this dc device. It includes a single axisymmetric coil surrounding a round pouring tube. An iron flux return donut surrounds the coil reducing the reluctance of the magnetic circuit. In practical application for controlling the flow of a steel stream exiting from a tundish, Shercliff's device does not allow sufficient space for the electric coil. As the resistance losses depend on the available cross-sectional area of the coil, as disclosed by Shercliff, the axisymmetric device also requires an impractical high power to operate. The coil is also located directly next to the pouring tube which will be at an elevated temperature. Cooling of the coil becomes difficult due to the heat transferred from the tube and internally generated by the resistive losses.
As described below, the present invention overcomes the disadvantages of the ac induced and externally applied current flow control devices cited earlier. The present invention also improves on and eliminates the deficiencies of the dc flow brakes known to date. Further objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing description.
Therefore, it is an object of the present invention to provide an electromagnetic flow control device that does not require high frequency alternating current for operation. It is a further object of the present invention to provide an electromagnetic flow control device that does not require any kind of special internal passage member to direct the flow. It is a further object of the present invention to provide an electromagnetic flow control device that is compact in size, readily manufacturable and is low in cost. It is another object of the present invention to provide an electromagnetic flow control device that does not require high power for operation.